The invention relates to a brake booster and more specifically to a rapid-actuation brake booster that can be applied in particular to motor vehicles.
In automotive braking systems, there is generally a perceptible dead travel at the onset of braking as the brake pedal is actuated and during which the driver depresses the brake pedal without any hydraulic pressure being effectively introduced into the vehicle braking circuit.
Systems do exist that reduce this dead travel. One such system is the one described in French Patent Application No. FR 2 856 363.
That system comprises, as depicted in FIGS. 1 and 2:
a casing 2 of longitudinal axis X containing a skirt 6 mounted such that it can slide axially in a leaktight manner in the casing and delimiting a low-pressure first chamber 8 known as the front chamber and a variable-pressure second chamber 10 known as the rear chamber;
a piston 12 secured to the skirt 6;
a three-way valve 26 which, under the control of a control rod 28, can be used to isolate the front and rear chambers from one another, to place them in communication with one another, or to place the rear chamber at a high pressure with respect to the low pressure such as atmospheric pressure;
a sleeve 54 sliding axially inside the piston 12, an annular face 48 of this sleeve acting as a first valve seat, known as the equalizing seat, for the three-way valve 26;
a distributor-plunger 32 which has an annular face 50 acting as a second valve seat, known as the inlet valve seat, for the three-way valve 26;
a first key 84 passing at right angles to the axis X through two diametrically opposed openings 86 in the piston and two diametrically opposed slots 88 in the sleeve 54. The key 84 is fixedly mounted in the pneumatic piston 12; and
a second key 90 passing at right angles to the axis X through the piston and the sleeve 54, this second key acting as a return axial end stop for the distributor-plunger 32.
The way in which a booster such as this works is as follows:
In the rest position (FIGS. 1 and 2), the valve shutter 46 is away from the first valve seat or equalizing valve 48 (borne by the sleeve 54) and places the front chamber 8 in communication with the rear chamber 10. The valve shutter 46 rests against the second inlet valve seat 50 thus isolating the rear chamber from atmospheric pressure.
At the start of a braking phase, as the driver depresses the brake pedal, the control rod 28 is moved axially forward, the valve shutter 46 then presses against the first valve seat 48, isolating the rear chamber from the front chamber and moves away from the second seat 50 allowing the rear chamber to be supplied with air at atmospheric pressure. Because of the pressure difference between the front chamber and the rear chamber, the skirt 6 and the piston 12 are moved forward. The first valve seat 48 borne by the sleeve 54 is stationary as long as the clearance C between the key 84 and the front end of the second slots 88 has not been taken up.
The spring 58 keeps the sleeve 54 in a set axial position with respect to the casing of the booster as long as the piston 12 has not covered a set travel C. The hydraulic piston of the master cylinder is pushed by the pneumatic piston which itself carries along the auxiliary piston 126 which moves away from the sensor. When the pressure in the master cylinder is above a set pressure high enough for the auxiliary piston 126 to be able to overcome the force of the jump spring 138, the auxiliary piston is pushed back toward the sensor until it comes into contact therewith and then passes on the reaction of the hydraulic circuit to the brake pedal.
When the pneumatic piston 12 has covered the travel C (see FIG. 2), the front face 100 of the key 84 which is fixed with respect to the pneumatic piston comes to rest against the front end of the slots 88 of the sleeve. The sleeve is then axially tied to the movement of the piston. The valve shutter 46 comes into contact with the inlet valve seat 50 and interrupts the supply of air at atmospheric pressure to the rear chamber. The driver has then to depress the brake pedal further in order to increase the intensity of braking.
The travel C is preferably chosen so that it corresponds to the dead travel of the master cylinder, that is to say with the travel that the hydraulic piston needs to cover within the master cylinder in order to begin to cause the pressure of the brake fluid in the brakes to rise. As a result, the driver feels through the pedal only the travel needed to close the equalizing valve and to open the inlet valve and does not feel the dead travel of the master cylinder. Driver comfort is thus improved, because the driver has the impression of immediate braking.
Thereafter, the system enters a phase of braking proper. Equalizing is achieved when the driver keeps the level of braking at a determined intensity. The valve shutter 46 is then in contact with the equalizing seat 48 and the inlet seat 50, thus interrupting the supply of air at atmospheric pressure to the rear chamber.
The booster then enters a saturation phase which corresponds to the instant the booster no longer provides any additional boost, the pressure obtaining in the rear chamber being atmospheric pressure. The pneumatic piston can no longer move forward axially under the action of the movement of the skirt 6. As a result, the sleeve 54 and the equalizing seat 48 are immobile. Any additional braking force applied by the driver moves the control rod 28 and the inlet seat away from the valve shutter 46 although this additional force is no longer boosted. The increase in pressure in the master cylinder is equal to the additional force provided by the driver, divided by the cross section of the master cylinder piston.
At the end of braking, the driver releases the brake pedal at least partially. The distributor-plunger is then carried rearward with the control rod. The inlet valve seat 50 comes into contact with the valve shutter 46 to interrupt the supply of air at atmospheric pressure to the rear chamber and carries the shutter 46 away from the equalizing seat 48, placing the front chamber 8 and the rear chamber 10 in communication. The pressures across the skirt 6 then equalize and this skirt returns to the rest position under the action of the return spring. The rest position of the pneumatic piston 12 is set by the second key 90 resting against the annular bearing surface 119 of the casing 2. The distributor-plunger also returns to rest coming into abutment against the key 90. The travel C between the front end of the slot 88 and the front face 100 of the key 84 is reestablished under the action of the sleeve return spring 58. The booster is once again in a position ready to operate with minimal actuating travel.
In a system such as this it is found that it is necessary to have a peak load applied to the pedal when the key 84 has covered the travel C and comes into contact with the front end of the slots 88 in the sleeve 54. The purpose of the invention is to diminish this transition which is perceptible at the brake pedal.
The invention therefore relates to a brake booster which comprises:
a casing of given longitudinal axis;
an assembly comprising a skirt and a pneumatic piston which is mounted to slide in a leaktight manner in the casing along the longitudinal axis, said skirt-and-piston assembly dividing the interior space of the casing into a low-pressure front chamber and a variable-pressure rear chamber;
a three-way valve actuated by a control rod mounted in a longitudinal passage pierced in the pneumatic piston, the said control rod being connected via a first longitudinal end to a brake pedal;
a distributor-plunger that can move, in the said piston, along the said longitudinal axis under the control of a second end of the said control rod, the said distributor-plunger comprising a sensor for applying the force of the control rod to a hydraulic piston of a master cylinder via a reaction device, the said skirt-and-piston assembly transmitting a pneumatic boost force to the hydraulic piston of the master cylinder;
a sleeve mounted to slide in a leaktight manner in the pneumatic piston along the said longitudinal axis over a set first travel; and
a securing means capable axially of connecting the said sleeve to the pneumatic piston when the said pneumatic piston has covered the set travel.
The three-way valve comprises a first valve seat and a second valve seat and a valve shutter intended to be pressed against the first and/or second valve seat. The second valve seat is borne by a first longitudinal end of the distributor-plunger. The first valve seat is borne by a first longitudinal end of the said sleeve.
According to the invention, the securing means comprises a device for gradually moving the sleeve according to the movement of the piston.
As a preference, the gradual movement device comprises a face that is inclined with respect to the said longitudinal axis and it is intended to transmit to the said sleeve a movement force along the longitudinal axis as supplied by the piston.
According to one embodiment of the invention, the said inclined face belongs to a front face of a key. This key has a first end which rests against a part secured to the piston or against the piston itself, and a second end which rests against the casing of the booster or against a part which itself rests against the casing of the booster.
In this embodiment, the first end or the second end of the key is fixed by an articulation to the part against which it rests.
Provision will then advantageously be made for the first end to be fixed to the piston by the said articulation and for the second end to have a curved portion which rests against the casing of the booster or against a part which itself rests against the casing of the booster.
As a preference, the key passes through the walls of the sleeve transversely to the said axis via slots, the inclined face of the key being intended to rest against front ends of the slots of the sleeve.
Under braking, provision will advantageously be made for the inclined face of the key to rest against the front ends of the two slots of the sleeve.
The invention also relates to an alternative form of embodiment in which there is at least one rod which passes at right angles to the said longitudinal axis through the walls of the sleeve via openings of which at least one wall is inclined with respect to the said longitudinal axis. Control means control the longitudinal movement along the inclined wall of the rod and allow an axial movement of the piston to be transmitted to the sleeve via the said rod.
According to this alternative form of embodiment, the said control means comprises a mitered part that has at least one face that is inclined with respect to the said longitudinal axis in the same direction as the inclined walls of the openings in the sleeve.
Provision will then advantageously be made for the inclined walls of the openings in the sleeve to make a first angle (X) with the longitudinal axis (X). The inclined face of the mitered part (79) makes a second angle (β) with the longitudinal axis. In such an instance, provision will be made for the first angle (X) to be greater than the second angle (β).
Furthermore, provision may also be made for the device according to the invention to comprise two rods that are parallel and symmetric with one another with respect to the longitudinal axis. The mitered part then has a shape that is symmetric about the said axis.
According to one embodiment of this alternative form, the two rods are made as a single piece and are joined together by a spring joint.
The invention also relates to another alternative form of embodiment according to the invention, whereby a key passes transversely through the sleeve via apertures. These apertures have a face that is inclined with respect to the said longitudinal axis. This inclined face rests against front ends of the said apertures. An axial movement of the said key, brought about by the movement of the piston, then causes the key to move vertically.
This alternative form of embodiment will then preferably provide a bearing part that has an inclined face against which one end (81) of the key rests.
Advantageously, the inclined face of the bearing part is substantially parallel to the inclined face of the said key.
The invention also relates to an additional alternative form of embodiment in which the sleeve has an aperture that is inclined with respect to the said longitudinal axis and a pin. A first end of this pin can move in this aperture and a second end can slide in a cavity in the piston.
Provision will then be made for the sleeve to be able to rotate in order to allow the first end of the pin to slide in the aperture.